1. Field of the Invention
The present invention relates to a heating device and a heating method. In addition, the present invention relates to an image forming apparatus using the heating device.
2. Discussion of the Related Art
Heating devices have been used for fixing devices of electrophotographic image forming apparatus such as copiers, printers, facsimiles and multifunctional image forming apparatus having copying, printing and facsimileing functions. Recently, such fixing devices use plural heaters or a heater having a relatively large capacity to rapidly raise the temperature of the heating members such as heat rollers.
Particularly, the heating capacity is greater in high speed image forming apparatus than that in medium speed image forming apparatus, and therefore the incoming current seriously increases in such high sped image forming apparatus when the heater achieves a lighting state (i.e., activated state), resulting in occurrence of large voltage variation in the image forming apparatus. Such voltage variation causes a flickering problem in that lamps set in a room flicker when such a high speed image forming apparatus is also set in the room, thereby making the persons working in the room uncomfortable.
In attempting to avoid the voltage variation causing the flickering problem, image forming apparatus using a controlling method (hereinafter referred to as a soft start controlling method) such that power application is gradually performed on the heater using phase controlling instead of full power application to control the incoming current are provided.
However, even when such a soft start controlling method is used, the incoming current seriously increases, resulting in occurrence of great voltage variation, if the plural heaters of the fixing device of the image forming apparatus are activated at the same time. In addition, by performing such phase controlling, a harmonic component is generated in the supply current. Therefore, when plural heaters are activated at the same time, a strong harmonic current is caused. In this case, problems in that the condenser of a power company is excessively heated and/or the breaker of the power company causes an error tend to be caused.
In attempting to solve the problems, a published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 09-305059 discloses an image forming apparatus having a fixing device in which plural heaters are subjected to ON/OFF controlling at the respective control cycles such that the control cycles thereof are differentiated by a predetermined cycle in such a manner that the plural heaters do not achieve the OFF state at the same time, and in addition the OFF timing of the plural heaters is subjected to variable control depending on the detected temperature of the image forming apparatus within a range of half the control cycles so that the control cycles of the heaters are not overlapped.
However, in the image forming apparatus disclosed in JP-A 09-305059, there is a case where plural heaters achieve an activated state at the same time. As a result, a strong harmonic current is caused, resulting in occurrence of the problems in that the condenser of a power company is excessively heated and/or the breaker of the power company causes an error.
Because of these reasons, a need exists for a heating device in which plural heaters are subjected to heating control with hardly causing the great voltage variation and strong harmonic current problems.